1. Field of the Invention
The present invention relates to an organic electroluminescence element (which may be referred to as an organic EL element hereinafter). More specifically, the present invention relates to an organic EL element using a triplet exciton of an organic luminescence material (host material).
2. Description of the Related Art
Hitherto, organic EL elements wherein an organic luminescence layer is arranged between electrodes have been eagerly researched and developed for the following reasons and the like.
(1) Since these elements are completely solid, they are easy to handle and produce.
(2) Since they can emit light by themselves, no light emitting members are necessary.
(3) Since they can be clearly watched, they are suitable for display.
(4) They permit full color display easily.
The luminescence mechanism of such organic EL elements generally makes use of a luminescence phenomenon, which is energy conversion phenomenon caused when a fluorescent molecule in a singlet excited state (which may be referred to a S1 state) in an organic luminescence medium is transited to a ground state radially.
A fluorescent molecule in a triplet excited state (which may be referred to a T1 state) in an organic luminescence medium can be supposed. However, radiative transition to a ground state is forbidden; therefore, such a molecule is gradually transited from the triplet excited state to some other state by non-radiative transition. As a result, no fluorescence is emitted but thermal energy is radiated.
Here, singlet and triplet mean multiplicity of energy decided by combination of total spin angular momentum and total orbital angular momentum of a fluorescent molecule. Specifically, a singlet excited state is defined as an energy state in the case that a single electron is transited from a ground state, where no unpaired electrons are present, to a higher energy level without changing the spin state of the electron. A triplet excited state is defined as an energy state in the case that a single electron is transited to a higher energy level while the spin state of the electron is made reverse.
Needless to say, luminescence in a triplet excited state defined as above can be observed if the luminescence is caused at a very low temperature, for example, at a liquefaction temperature of liquid nitrogen (xe2x88x92196xc2x0 C.). However, this temperature is not a practical temperature, and the amount of the luminescence is only a little.
By the way, the total efficiency of luminescence from any conventional organic EL element is related to recombination efficiency (xcfx86rec) of injected charged carries (electrons and holes), and the probability (xcfx86rad) that generated excitons cause radiative transition. Therefore, the total efficiency (xcfx86el) of luminescence from the organic EL element can be represented by the following equation:
xcfx86el=xcfx86recxc3x970.25xcfx86rad 
The coefficient (0.25) of xcfx86rad in the equation is decided from the matter that the probability that singlet excitons are generated is regarded as xc2xc. Therefore, even if recombination and radiative attenuation of excitons are caused with a probability coefficient of 1, the theoretical upper limit of luminescence efficiency of the organic EL element is 25%.
As described above, in any conventional organic EL element, triplet excitons cannot be substantially used and only singlet excitons cause radiative transition. Thus, a problem that the upper limit of the luminescence efficiency is low arises.
Thus, literature 1 xe2x80x9cJpn. J. Appl. Phys., 38 (1999) L1502xe2x80x9d discloses that even at room temperature, triplet excitons (triplet excited state) of an organic luminescence material (host material) are used to transfer energy from the triplet excitons to a phosphorescent dopant, so as to generate a fluorescent luminescence phenomenon. More specifically, the literature 1 reports that a fluorescent luminescence phenomenon is caused in an organic EL element comprising an organic luminescence layer composed of 4,4-N,N-dicarbazolylbiphenyl represented by the following formula (6) and an Ir complex, which is a phosphorescent dopant. 
However, the half-life of the organic EL element described in the literature 1 is below 150 hours, and the usefulness of the organic EL element is insufficient.
Thus, the inventor made eager investigations. As a result, the following has been found: the glass-transition temperature of 4,4-N,N-dicarbazolylbiphenyl is as low as less than 110xc2x0 C.; therefore, if the biphenyl is combined with an Ir complex, crystallization is easily caused in the organic luminescence layer comprising the combination to make the life of an organic EL element short.
Incidentally, in the present situation, a demand that the heat-resistance of organic EL elements for cars should be made higher has been increasing in light of environment inside cars in summer.
Thus, an object of the present invention is to provide an organic EL element which makes it possible to use triplet excitons of an organic luminescence material (host material) even at room temperature to emit fluorescence (including phosphorescence); has a practical life span; and has a superior heat-resistance.
[1] According to the present invention, provided is an organic EL element comprising:
an anode layer,
a cathode layer, and
an organic luminescence layer therebetween, the organic luminescence layer having a carbazole derivative with a glass-transition temperature of 110xc2x0 C. or higher, and a phosphorescent dopant. Thus, the above-mentioned problems can be solved.
This organic EL element makes it possible to use the triplet exciton state of the organic luminescence material even at room temperature. Moreover, this element has a practical life, for example, a half-time of 300 hours or more, and has superior heat-resistance. Thus, this element can be sufficiently used as an organic EL element for car.
[2] In the organic EL element of the present invention, it is preferred that the carbazole derivative is at least one of compounds represented by the following general formulae (1) to (4): 
wherein Ar1 is a substituted or non-substituted aryl group having 6 to 50 nucleus carbon atoms; Ar2 to Ar7 are each independently a substituted or non-substituted aryl or arylene group having 6 to 50 nucleus carbon atoms; Ar2 and Ar3, Ar4 and Ar5, or Ar6 and Ar7 may be connected to each other through a single bond or through O, S or substituted or non-substituted alkylene as a connecting group; and each of repetition numbers m and n is an integer of 0 to 3,
wherein R1 to R6 are each independently a hydrogen or halogen atom, an alkyl, aralkyl, aryl, cycloalkyl, fluoroalkyl, amino, nitro, cyano, hydroxy, or alkoxy group; R7 and R8 are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; X1 and X2 are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; Y is a single bond, an alkyl, alkylene, cycloalkyl, aryl, or aralkyl chain; a repetition number p is an integer of 1 to 3.
wherein Ar8 to Ar11 are each independently an aryl group having 6 to 50 nucleus carbon atoms which may be substituted with an alkyl, alkoxy or aryl group; Ar8 and Ar9, or Ar10 and Ar11 may be connected to each other through a single bond or through O, S or substituted or non-substituted alkylene as a connecting group; and R9 is an alkyl or alkoxy group, or a substituted or non-substituted aryl group having 6 to 18 nucleus carbon atoms. 
wherein Z is a trivalent nitrogen atom or an aromatic group; Ar12 to Ar14 are each independently a group represented by the following general formula (5) or an aryl group having 6 to 50 nucleus carbon atoms; and at least two of Ar12 to Ar14 are groups represented by the following general formula (5): 
wherein R10 to R21 are each independently an aryl group having 6 to 50 nucleus carbon atoms which may be substituted with an alkyl, alkoxy group having 1 to 6 carbon atoms, or a phenyl group; and groups adjacent to each other may form a cyclic structure; and a repetition number q is an integer of 0 to 3.
The organic EL element wherein this carbazole derivative is used as a host material in the organic luminescence layer makes it possible to use the triplet exciton state more effectively, and has a practical life span.
[3] In the organic EL element of the present invention, it is preferred that the carbazole derivative has at least two carbazole skeletons.
This carbazole derivative has a large triplet energy to make it possible to use the triplet exciton state more effectively even at room temperature (20xc2x0 C.), and has a practical life span.
[4] In the organic EL element of the present invention, it is preferred that the relationship of E1 greater than E2 is satisfied in which E1 represents the triplet energy of the carbazole derivative and E2 represents the triplet energy of the phosphorescent dopant.
This structure makes it possible to transfer the triplet energy of the carbazole derivative surely to the phosphorescent dopant, and to emit fluorescence using the triplet energy even at room temperature (20xc2x0 C.).
[5] In the organic EL element of the present invention, it is preferred that the triplet energy (E1) of the carbazole derivative is a value of 21,000 cmxe2x88x921 or more.
A triplet energy of 21,000 cmxe2x88x921 corresponds to a light wavelength of 488 nm. On the contrary, various phosphorescent dopants generally have a triplet energy which is equal to or less than the energy which 488 nm light has. Therefore, by using the carbazole derivative having such a large triplet energy as above, various phosphorescent dopants can be used.
Thus, by selecting an appropriate kind of the phosphorescent dopant for the carbazole derivative having such a large triplet energy as above, luminescence in green, yellow, orange, vermilion, red and the like can easily be obtained.
[6] In the organic EL element of the present invention, it is preferred that the carbazole derivative has a cyclic structure whose triplet energy is a value of 21,000 cmxe2x88x921 or more, and the cyclic structure contains an aromatic ring, a hetero ring, or combination thereof.
This carbazole derivative makes it possible to transfer the triplet energy of the carbazole derivative more effectively to the phosphorescent dopant. Specifically, if the carbazole derivative has a cyclic structure having a triplet energy of less than 21,000 cmxe2x88x921, the triplet energy is transferred to this cyclic structure so that the triplet energy transferred to the phosphorescent dopant may be reduced.
[7] In the organic EL element of the present invention, it is preferred that the phosphorescent dopant is a metal complex comprising at least one metal selected from the group consisting of Ir (iridium), Ru (ruthenium), Pd (palladium), Pt (platinum), Os (osmium) and Re (rhenium).
This structure makes it possible to transfer energy effectively from the triplet exciton of the carbazole derivative as a host material to the metal complex as the phosphorescent dopant.
[8] In the organic EL element of the present invention, it is preferred that at least one ligand of the metal complex has at least one skeleton selected from the group consisting of phenylpyridine, bipyridyl and phenanthroline skeletons.
The bulky and electron-withdrawing skeleton(s) contained in the molecule makes it possible to transfer energy effectively from the triplet exciton of the carbazole derivative to the metal complex.
[9] In the organic EL element of the present invention, it is preferred that a blend amount of the phosphorescent dopant is 0.1 to 30 parts by weight per 100 parts of the carbazole derivative.
This structure makes it possible to mix the phosphorescent dopant with the carbazole derivative uniformly, and transfer energy effectively from the triplet exciton of the carbazole derivative to the phosphorescent dopant.
[10] In the organic EL element of the present invention, it is preferred that a hole barrier layer, an electron injection layer, or combination thereof is arranged between the anode layer and the cathode layer, and the hole barrier layer and the electron injection layer comprise an alkali metal.
This structure makes it possible to drive the organic EL element at a lower voltage, and make the life span of the element longer.